Assuming you have the "premium" speaker system then either the amp is dead or you have a wiring problem between the head unit and the amp. See
viewtopic.php?f=45&t=73768 for some images of the speakers and amp I did when replacing the speakers.
I don't know how to test the functionality of the amp or really how to check the wiring but on the latter I imagine you can pull the head unit and at least check for continuity from there to the amp.
Yes the speaker foam can go, there are kits available to replace it, but I wouldn't think this would kill the sound entirely especially on 2 speakers at the same time. Any sound from the left dash speaker? If so the amp is likely ok.
The amps are not critical in the sense you can put in a new aftermarket head unit; new speakers; and a bit of wiring to run the speakers directly but the dash ones won't work.